A thermal conductivity cell used in a thermal conductivity detector is generally comprised of an electrically heated filament mounted in a cavity through which the carrier gas and sample gas eluting from a gas chromatograph column may be passed. As the concentration of the sample gas in the carrier gas changes, it varies the rate at which heat flows from the filament to the walls of the cavity and, consequently, the amount of electrical power required to keep the filament at a constant resistance or temperature. Accordingly, a measurement of this power can be used as an indication of the concentration of sample gas in the carrier gas flowing through the cavity.
In a U.S. patent application, Ser. No. 730,559, entitled "Modulated Fluid Detector", and filed on Oct. 7, 1976, in the name of David E. Clouser and myself, a thermal conductivity cell is disclosed that is comprised of two wafers having a groove in one or both abutting planar surfaces. A filament is mounted in an intermediate portion of the groove, and electrical conductors that are connected to respective ends of the filament are passed in sealed relationship through one of the wafers to points outside the cell. A number of passageways are provided in the other wafer for communicating from different points outside of the cell to predetermined points in the groove. Connections from these passageways to various points in the chromatograph are provided by bonding metal tubing in communicating relationship with the passageways.
As it is necessary to replace the cell from time to time because of filament failure or other defects, it is desirable that its initial cost be low and that its structure be such that it can be easily replaced in the field without special equipment. Making the wafers of the cell from ceramic material instead of metal would make its initial cost very low, but presently known techniques for bonding the metal tubes to the passageways in a ceramic wafer, while satisfactory for use in the factory, are so difficult to use in the field that it would be easier and more economical to throw away the expensive metal tubes along with the defective cell.